(1) Field of the Invention
The present invention generally relates to a shellcase body for use as part of an ammunition cartridge, which may be used with both rifles and pistols. In particular, the present invention is directed to a shellcase body and method for controlling the reflection of primer shockwaves.
(2) Description of the Related Art
Shellcase bodies typically have one of two general designs: straight and bottleneck. Bottleneck shellcase bodies include a shoulder portion that defines a bottleneck cross-section. Bottleneck shellcase bodies were developed to house larger amounts of propellants than their predecessor, the straight-walled shellcase. While bottleneck shellcases achieve the goal of greater propellant capacity, their internal geometry may cause problems with propellant ignition. Primer explosion shockwaves reflect off the shoulder to cause propellant throughout the shellcase to ignite. It is however possible that in an ill designed bottlenecked shellcase the shockwave reflections may be misguided and be detrimental to the overall performance level of the ammunition cartridge. A typical bottleneck design includes a frusto-conical portion disposed between a larger cylindrical portion containing propellant and a smaller cylindrical portion that contains a projectile.
Prior attempts have been made to define bottleneck shellcase shoulders with forms other than the most common frusto-conical section. However, previous designs have typically been limited by their own manufacturability and the availability of tools required to manufacture them. In addition, other previous designs typically fail to properly control the location of primer explosion shockwaves.
One previous design as disclosed in U.S. Pat. No. 6,523,475 includes a shoulder defined by an ellipse centered on the longitudinal axis of the shellcase. The ellipse foci are located at the origin of the primer explosion shockwave and just behind the base of the bullet. Unfortunately, this design suffers from multiple shortcomings. First, due to the modern state of computer-driven manufacturing operations, it is difficult to program shape cutting equipment with ellipsoidal shapes. Second, due to the internal nature of the elliptically defined shape, it will likely be difficult to ensure that shellcase manufacture will result in the desired ellipsoidal shape and not a slightly different ellipsoidal shape, which would counteract the anticipated performance gains. Third, the prior design does not appear to address how the ellipsoidal shellcase will headspace, i.e., fit, within a firearm chamber. Finally, the ellipsoidal shellcase of the prior design is designed to redirect the primer explosion shockwaves to a single point within the inner cavity of the shellcase. However, manufacturing tolerances inherent in common ammunition-manufacturing processes will make it difficult to achieve such precise redirection of the primer explosion shockwaves.
Referring now to FIG. 1, another previous design includes a shellcase body 20 having a straight sidewall 22 joined to a shoulder 24, which includes a curvature that is defined by a circular arc 26 having a center 28 that is positioned a distance D away from the longitudinal axis 30 of the shellcase. Straight sidewall 22 is joined to shoulder 24 at a tangent point 32 of circular arc 26, i.e., the straight sidewall defines a tangent line 34 that intersects the circular arc at the tangent point. Although the design of FIG. 1 is an improvement over previous designs, it too has shortcomings.
By joining straight sidewall 22 to shoulder 24 at tangent point 32, the curvature of the shoulder defined by circular arc 26 is too shallow. A shallow curvature causes primer explosion shockwaves 36, which originate at primer explosion 38, to reflect off shoulder 24 to an area 40 that extends into a neck portion 42 of shellcase body 20. Typically, neck portion 42 holds a projectile 44, which includes an aft end 46 that will likely be encroached by area 40. As a result, projectile 44 may become prematurely dislodged from the shellcase neck, i.e., before the propellant (not shown) contained in shellcase 20 is sufficiently ignited by the primer blast flame front and the concentration of the redirected primer explosion shockwaves 36.